1. Field of the Invention
The present invention relates generally to the field of robotic wrists. More specifically, the present invention discloses a robotic wrist using an electro-rheological fluid to control and adjust the degree of stiffness and compliance of the wrist.
2. Statement of the Problem
Many robotic tasks, including assembly, require local sensing and repositioning of the tool. This repositioning can be attributed to minor variations in the external working environment, such as inconsistent physical dimensions of parts and components. As a result, robotic joints need to be adjustable with "tuneable" stiffness, particularly if the device is to experience a range of tool lengths and payloads. For example, the wrist should be stiff when lifting a heavy load and compliant when doing delicate work.
The prior art contains several examples of robotic arms, manipulators, and joints. The prior art also contains examples demonstrating various uses of electro-rheological fluids. The following are a few examples:
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ Haaker, et al. 3,503,273 Mar. 31, 1970 Colston 3,904,042 Sept. 9, 1975 Muench 4,433,953 Feb. 28, 1984 ______________________________________
Cutkosky and Wright, "Position Sensing Wrists for Industrial Manipulators" (Robotics Institute, Carnegie-Mellon University, July 1982). PA0 Cutkosky and Wright, "Active Control of a Compliant Wrist in Manufacturing Tasks", Transactions of the ASME, Volume 108, pages 36-43 (Feb. 1986). PA0 "Electro-Rheological Fluids", informational brochure distributed by American Cyanamid Company, Wayne, N.J. PA0 Duclos, et al., "Fluids That Thicken Electrically", Machine Design (Jan. 21, 1988).
Haaker, et al. disclose a wrist joint for mounting the handle and tong means of a remote control master-slave manipulator of the type used by an operator to perform manipulative functions in some area remote from the operator (e.g. on the opposite side of a shielding wall). The device is a two degree of freedom wrist requiring two motors and cables to operate.
Colston discloses another example of a manipulator arm.
Muench discloses an industrial robot having a joint-free arm. The arm is formed as a hollow tube made from a material that will provide a degree of structural rigidity to enable the arm to support a work tool and provide controlled bending of the tube when the hollow interior of the tube is subjected to variations in pressure.
The articles by Cutkosky and Wright discuss compliant robotic wrists. Reinforced elastomeric spheres are used to float the gripper from the manipulator. The four spherical hydraulic actuators behave as adjustable springs by pressurizing the hydraulic fluid. Increasing hydraulic pressure increases stiffness of the wrist. The stiffness of the spheres can be adjusted to project the center of compliance over a range of several inches out from the upper platform of the wrist. Its 51/2 degrees of freedom make this device compliant in each direction except axial extension. As force is applied to the end effector, the stiffness monotonically increases. Thus, the wrist is very sensitive to light loads. For heavier loads the sensitivity reduces and the deflections do not become excessive. This approach has the disadvantage of requiring extensive external hydraulic components (i.e. a pump, reservoir, piping, and solenoid control values) to maintain and regulate hydraulic pressure to each of the spheres.
Electro-rheological fluids have the unique property of becoming more viscous or completely solidifying when placed in an electromagnetic field. The effect is extremely rapid, occurring within milliseconds, and is completely reversible. Fluids exhibiting this effect have been known since the late 1940's. These fluids have sometimes been referred to as "electro-viscous" fluids. Early electro-rheological fluids were made by suspending silica gel in a non-conducting oil, or by suspending starch particles in oil. A number of electro-rheological fluids made of liquid suspensions of fine polymer particles in non-conducting oil are commercially available from American Cyanamid Company of Wayne, N.J. The brochure entitled "Electro-Rheological Fluids", distributed by American Cyanamid Company, provides a general discussion of the properties and historical development of electro-rheological fluids. Typical applications for electro-rheological fluids include electrically controlled brakes, clutches, hydraulic valves, actuators, shock absorbers and dampers. Further general discussion of electro-rheological fluids is provided in Duclos, et al., "Fluids That Thicken Electrically", Machine Design (Jan. 21, 1988).
3. Solution to the Problem
None of the prior art references use electro-rheological fluids to adjustably control the stiffness of robotic wrist. This solution dramatically reduces that complexity of the robotic wrist by eliminating the need for the hydraulic components used to maintain and regulate fluid pressurization of the elastomeric spheres in a robotic wrist such as that disclosed by Cutkosky and Wright.